A near field communication (NFC) device generally includes a chain for receiving a signal connected to an antenna.
FIG. 1 shows a receive chain Rx connected to an antenna ANT and including an antenna coupling stage 1 generally including connecting capacitors and a balun.
The antenna coupling stage 1 decomposes the signal received by the antenna ANT into two signals in phase opposition with respect to each other.
An input of stage 1 is connected to the antenna ANT and each of the two outputs of stage 1 (e.g., providing the two signals in phase opposition with respect to each other) is connected to a different input of a variable gain amplifier 2 including an amplifier, for example transistors connected as followers.
Each of the outputs of the amplifier 2 is connected to a different input of an analog-to-digital converter (ADC) 4 by the intermediary of an anti-aliasing filter 3.
The amplifier 2 is driven by a variable gain controller 5, which may be referred to as an automatic gain control (AGC).
The controller 5 is connected to the converter 4 and includes a state machine that controls the value of the gain of the amplifier 2 according to the resolution of the converter 4 in such a way that the amplitude of a signal received by the converter 4 is contained within the maximum dynamic range of the converter 4.
The converter 4 drives the controller 5.
During a first step, the amplifier 2 determines the maximum gain permissible by the converter 4.
The maximum gain obtained is maintained constant as long as the intensity of the signal has not been modified, that is to say as long as the distance between the transmitter of the signal and the antenna ANT of the receiver Rx remains constant.
However, the signals received by the amplifier 2 include a common mode voltage offset.
FIG. 2 shows a signal S1 received on one of the inputs of the amplifier 2 exhibiting an offset DEC1 (e.g. common mode voltage offset) and the amplitudes ASUP and AINF representing the dynamic range of the converter 4.
The offset of the signal received on each input can be of different value.
It is assumed that the gain of the amplifier has been determined such that the signal S1 amplified by the amplifier 2 is contained within the dynamic range of the converter 4.
Let S2 be the result of signal Si having been amplified by the amplifier 2.
It is observed that a part of the signal S2 is not contained within the dynamic range of the converter 2. The multiplication of the offset DEC1 of the signal S1 by the gain of the amplifier 2 has created an offset DEC2 of the signal S2 greater than the offset DEC1.
The amplitude of the signal S2 at the output of the amplifier 2 is greater than the maximum dynamic range of the converter 4.
Only the part of the signal S2 contained within the dynamic range of the converter 4 is converted into a digital signal. This results in a large error in the digital signal obtained at the output of the converter 4 with respect to the signal received by the antenna ANT.
Connecting capacitors, placed at the output of the amplifier 2, make it possible to bias the signals at a different level. However, the offset signal is transmitted to the amplifier 2 and can saturate the latter.
Another known solution includes oversizing the analog-to-digital converter so as to increase its dynamic range.
However, such a converter gives rise to an increase in energy consumption, of the area of the amplifier circuit and of the cost.
There is a need for a variable gain amplifier compensating for the offsets of the signals received on each of its inputs.